In conventional machine tools, sensing operations such as tool breakage sensing and tool wear sensing are performed by sensing the thrust load applied to a spindle of a machine tool. Thus cutting control of the machine tool may be adapted to the detected present conditions.
In the above-mentioned conventional methods for sensing the thrust load, the power of a spindlehead feed motor or the thrust load applied to a feed screw is taken as (substituted for) the thrust force applied to the spindle.
However, in case of sensing the power of the spindle-head feed motor or the thrust load applied to the feed screw, not only tool cutting resistance but also other frictional resistances such as frictional resistance against rotation within bearing support for a ball screw and sliding friction resistance applied to the spindlehead are also sensed at the same time. Accordingly, it is necessary to calculate a value representative of tool wear/breakage, e.g. by subtraction of frictional forces from the sensed power value. Thus, the sensing of tool wear/breakage tends to have low accuracy.